A charge-coupled device (CCD), also generally known as a charge transfer device, is a dynamic device which moves charge along a predetermined path under the control of clock pulses. Charge-coupled devices are used in a variety of different types of devices and applications, such as signal processing, memories and imaging. Basically, there are two types of area CCDs, interline CCDs and full frame CCDs.
In imaging applications, interline CCDs are used primarily in image sensors used for capturing movie or video-type images. With these image sensors, a plurality of photosensors are each coupled to an interline CCD. A shield covers the interline CCD and most of the image sensor except for the photosensors. Typically, the photosensors only occupy about twenty to twenty-five percent of the surface area of the image sensor exposed to the image. Accordingly, when an image is exposed to the image sensor, a substantial portion of the image is lost to areas of the image sensor which are not photosensitive.
The advantage of interline CCD image sensors is the separation of the photocollection function from the charge transfer function. This separation allows each photosensor to form charge packets in proportion to the intensity of light exposed to the particular photosensor which can be quickly shifted out to the interline CCD for transfer allowing the photosensors to begin storing charge packets for the next image. Thus, images can be captured and transferred out in a rapid sequence, making interline CCD image sensors particularly useful for imaging apparatus for making movies or videos.
In other imaging applications, such as still photography, full frame CCD image sensors are used. In full frame CCD image sensors, the CCDs include a photosensitive layer which covers all or virtually all of the surface area which is to be exposed to the image to be captured. Accordingly, with full frame CCD image sensors, all or virtually all of an image exposed to the sensor can be scanned or captured. Unlike interline CCD image sensors, the photocollection function is not separated from the charge transfer function in these sensors. Therefore, once the image has been captured by the full frame CCD image sensor, the photosensors must be shielded until the full frame CCDs can shift out the stored charge packets for processing. Any exposure of the photosensors to light during the transfer process will result in additional charge being stored in the charge packets as they are being shifted out. This additional charge will smear the resulting image. Thus, full frame CCD image sensors are able to capture more of the image, than interline CCD image sensors, but do not operate as quickly because they must perform the photocollection and charge transfer functions in sequence.
One of the major problems with interline CCD imager sensors is the loss of a portion of the image because of the particular architecture of the sensor. To overcome this problem the process of dithering was developed. With dithering the image is scanned and then either the image or the photosensitive areas of the interline CCD image sensor are shifted so that the image sensor can scan or capture another portion of the image. To obtain a higher resolution image, several scans of the image are generally needed. Heretofore, the dithering process has not been used with full-frame CCD image sensors because these sensors are already able to capture all or virtually all of the image. Thus, there would be no need to employ the dithering process.
A problem with full frame CCD imaging sensors has been the trade-off between capturing the full resolution of an image and the cost of a full frame CCD image sensor which can perform one-to-one imaging. For example, a film negative typically consists of an image which is 3,000.times.2,000 pixels. To avoid losing any resolution of the image, an image sensor which has 3,000.times.2000 pixels would be needed. Unfortunately, the cost of such an image sensor to perform one-to-one imaging is prohibitively expensive. If an image sensor which had 1,500.times.1,000 pixels was used, the cost of the apparatus would be reduced making the apparatus more commercially marketable, however there would be a significant loss of resolution in any image captured because one pixel of the image sensor would capture or scan four pixels of the image on the film negative.